Excerpt
A ratio is simply a comparison between two different things. For example, if a pet store has 25 animals and we want to know the ratio of cats to dogs, we would count the cats and dogs. If we count 15 cats and 10 dogs, we would say that the ratio of cats to dogs is 15 to 10. We could also represent this ratio by saying the ratio of cats to dogs is 15:10. This is called odds notation.

It is important to note the order of your terms when talking about ratios. Since we are saying the ratio of cats to dogs, we put the number of cats first. The number of dogs is second because it is the second term. It is what we are comparing the number of cats to.